Battery separator, and battery separator manufacturing method

ABSTRACT

A battery separator includes a porous membrane A with a thickness of less than 10 μm including a polypropylene resin, and a porous membrane B laminated thereon including a heat resistant resin and inorganic particles or cross-linked polymer particles, wherein the porous membrane A satisfies a specific range of thickness, average pore size, and porosity, and the entire battery separator satisfies a specific range of thickness, peeling strength at the interface between the porous membrane A and the porous membrane B, and difference in air resistance between the entire battery separator and the porous membrane A.

TECHNICAL FIELD

The present invention relates to a battery separator comprising a heatresistant resin layer and a polypropylene porous membrane having athickness of less than 10 μm, and particularly to a battery separatoruseful as a high-performance and low-cost-oriented lithium ion batteryseparator.

BACKGROUND ART

Thermoplastic resin microporous membranes have been widely used, forexample, as a material for separation, selective permeation, andisolation of substances: e.g., battery separators used in a lithium ionsecondary battery, nickel-hydrogen battery, nickel-cadmium battery, andpolymer battery; separators for an electric double layer capacitor;various filters such as a reverse osmosis filtration membrane,ultrafiltration membrane, and microfiltration membrane;moisture-permeable waterproof clothing; and medical materials. Inparticular, polyolefin microporous membranes have been suitably used asa lithium ion secondary battery separator, because they are not onlycharacterized by having excellent electrical insulating properties,having ion permeability due to electrolyte impregnation, and havingexcellent electrolyte resistance and oxidation resistance, but also havesuch a pore-blocking effect that excessive temperature rise issuppressed by blocking a current at a temperature of about 120 to 150°C. in abnormal temperature rise in a battery. However, when thetemperature continues to rise for some reason even after pore blocking,membrane rupture can occur at a certain temperature due to decrease inviscosity of molten olefin constituting the membrane and shrinkage ofthe membrane. In addition, when left at a constant high temperature,membrane rupture can occur after the lapse of a certain time due todecrease in viscosity of molten polyolefin and shrinkage of themembrane. This phenomenon is not a phenomenon that occurs only whenpolyolefin is used, and also when other thermoplastic resins are used,this phenomenon is unavoidable at or higher than the melting point ofthe resin constituting the porous membrane.

In particular, separators for a lithium ion battery are highlyresponsible for battery properties, battery productivity, and batterysafety, and required to have excellent mechanical properties, heatresistance, permeability, dimensional stability, pore-blockingproperties (shutdown properties), melt rupture properties (meltdownproperties), and the like. Accordingly, various studies to improve heatresistance have been conducted until now. Fluororesins which have bothheat resistance and oxidation resistance have been suitably used as aheat resistant resin layer.

Further, to increase battery capacity, it is expected that membraneswill become thinner and thinner in order to increase the area not onlyof electrodes but also of a separator that can be loaded into acontainer. When a porous film becomes thinner, it tends to be deformedin the planar direction, and accordingly a heat resistant resin layermay be peeled off during processing of a battery separator, a slittingprocess, or a battery assembly process, which makes it difficult toensure safety.

Further, to achieve cost reduction, it is expected that the speed willbe faster in a battery assembly process, and the present inventorspresume that there will be a demand for even higher adhesion towithstand high-speed processing, by which troubles such as peeling-offof a heat resistant resin layer hardly occurs even in such high-speedprocessing. Furthermore, it is expected that there will be an increasingdemand for higher processability (lower curling properties) in a batteryassembly process in the future.

Patent Document 1 discloses a lithium ion secondary battery separatorobtained by direct application of a polyamide-imide resin to apolyolefin porous membrane with a thickness of 25 lam to a thickness of1 μm and immersion in water at 25° C., followed by drying.

In Patent Document 1, significant increase in air resistance wasunavoidable, and, in addition, curling was large and not satisfactory.

Patent Document 2 discloses an electrolyte-supported polymer membraneobtained by immersion of a nonwoven fabric with an average thickness of36 μm comprising aramid fibers in a dope containing a vinylidenefluoride copolymer which is a heat resistant resin, and drying.

Patent Document 3 discloses a composite porous membrane obtained byimmersion of a polypropylene microporous membrane with a thickness of25.6 μm in a dope mainly composed of polyvinylidene fluoride which is aheat resistant resin, followed by the process of a coagulation bath,washing with water, and drying.

As in Patent Document 2, in a method in which coating is performed bydipping (immersing) a nonwoven fabric comprising aramid fibers in a heatresistant resin solution, a heat resistant porous layer is formed insideand on both surfaces of the nonwoven fabric, and accordingly most ofcontinuous pores inside the nonwoven fabric will be blocked;consequently, significant increase in air resistance is unavoidable, andbesides a pore-blocking function, the most important function thatdetermines safety of a separator, cannot be provided.

In addition, nonwoven fabrics are difficult to thin as compared topolyolefin porous membranes, and therefore are not suitable for increasein battery capacity which is expected to progress in the future.

Also in Patent Document 3, a heat resistant porous layer is similarlyformed inside and on both surfaces of a polypropylene microporousmembrane. As in Patent Document 2, significant increase in airresistance is unavoidable, and it is difficult to obtain a pore-blockingfunction.

Patent Document 4 discloses a separator having a heat resistant porouslayer comprising para-aramid obtained in such a manner that, when asolution of para-aramid resin which is a heat resistant resin is appliedto a polyethylene porous film with a thickness of 25 μm, thepolyethylene porous film is impregnated in advance with a polar organicsolvent used in the heat resistant resin solution in order to avoidsignificant increase in air resistance, and after the heat resistantresin solution is applied, the polyethylene porous film is made into awhite opaque membrane in a thermo-hygrostat set at a temperature of 30°C. and a relative humidity of 65%, and then washed and dried.

In Patent Document 4, there is no significant increase in airresistance, but adhesion between the polyethylene porous film and theheat resistant resin is extremely low. Particularly when the thicknessof the polyethylene porous film is less than 10 μm, the film tends to bedeformed in the planar direction, and accordingly the heat resistantresin layer may be peeled off in a battery assembly process, which makesit difficult to ensure safety.

Patent Document 5 discloses a composite porous membrane obtained in sucha manner that a propylene film is coated with a polyamide-imide resinsolution and passed through an atmosphere at 25° C. and 80% RH over 30seconds to obtain a semi-gel like porous membrane; then a polyethyleneporous film with a thickness of 20 μm or 10 μm is laminated onto thesemi-gel like porous membrane, immersed in an aqueous solutioncontaining N-methyl-2-pyrrolidone (NMP), and then washed with water anddried. However, curling was not satisfactory.

Further, in Patent Document 5, there is no significant increase in airresistance, but adhesion between the polyethylene porous film and theheat resistant resin is extremely low. Similarly to Patent Document 4,particularly when the thickness of the polyethylene porous film is lessthan 10 μm, the heat resistant resin layer may be peeled off, whichmakes it difficult to ensure safety.

As described above, in a battery separator in which a heat resistantresin layer is laminated on a porous membrane based on polyolefin or thelike that serves as a substrate, when the heat resistant resin isinfiltrated into the porous membrane that serves as a substrate toimprove the adhesion of the heat resistant resin layer, the amount ofair resistance increase is large. When the infiltration of the heatresistant resin is reduced, the amount of air resistance increase can bekept small, but the adhesion of the heat resistant resin layerdecreases. In particular, with separators becoming thinner, in light ofthe speed-up in a battery assembly process, it becomes difficult toensure safety and productivity, the demand for which will beincreasingly greater. In particular, as the thickness of the polyolefinporous membrane that serves as a substrate becomes thin, it becomes moredifficult to ensure low curling properties.

In other words, there has not been a battery separator that has lowcurling properties and a balance between adhesion of a heat resistantresin layer and the amount of air resistance increase, in which batteryseparator the thickness of a polyolefin porous membrane that serves as asubstrate is less than 10 μm. As the thickness of a porous membranebased on polyolefin or the like that serves as a substrate becomesthinner, it becomes more and more difficult to achieve a balance betweenadhesion of a heat resistant resin layer and the amount of airresistance increase.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP 2005-281668 A

Patent Document 2: JP 2001-266942 A

Patent Document 3: JP 2003-171495 A

Patent Document 4: JP 2001-23602 A

Patent Document 5: JP 2007-125821 A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The present inventors, on the assumption that battery separators becomethinner and thinner and their cost is further reduced in the future, aimto provide a battery separator that has low curling properties and abalance between excellent adhesion of a heat resistant resin layer and asmall amount of air resistance increase, and is suitable for increase inbattery capacity, excellent ion permeability, and high-speedprocessability in a battery assembly process; in particular, a batteryseparator suitable as a lithium ion secondary battery separator.

Means for Solving the Problems

To solve the problems described above, the present invention has thefollowing constitution.

(1) A battery separator, comprising: a porous membrane A comprising apolypropylene resin, and a porous membrane B laminated thereoncomprising a heat resistant resin and inorganic particles orcross-linked polymer particles, wherein the porous membrane A satisfiesexpressions (a) to (c), and the entire battery separator satisfiesexpressions (d) to (f).

t(A)<10 μm  Expression (a)

wherein t (A) is a thickness of the porous membrane A;

0.01 μm≦<R (A)≦1.0 μm  Expression (b)

wherein R (A) is an average pore size of the porous membrane A;

30%≦V(A)≦70%  Expression (c)

wherein V (A) is a porosity of the porous membrane A:

t(T)≦13 μm  Expression (d)

wherein t (T) is a thickness of the entire battery separator;

F(A/B)≧1.0 N/25 mm  Expression (e)

wherein F (A/B) is a peeling strength at the interface between theporous membrane A and the porous membrane B; and

20≦Y−X<100  Expression (f)

wherein X is an air resistance (sec/100 cc Air) of the porous membraneA, and

Y is an air resistance (sec/100 cc Air) of the entire battery separator.

(2) The battery separator according to (1), wherein the batteryseparator has an air resistance of 50 to 600 sec/100 cc Air.(3) The battery separator according to (1) or (2), wherein the heatresistant resin comprises a polyamide-imide resin, a polyimide resin, ora polyamide resin.(4) The battery separator according to (1) or (2), wherein the heatresistant resin comprises a fluororesin.(5) A process for producing the battery separator according to any oneof (1) to (4), comprising the following steps (i) and (ii).

Step (i): applying a varnish comprising a heat resistant resin andinorganic particles or cross-linked polymer particles to a porousmembrane A comprising a polypropylene resin, and then passing the porousmembrane A through a low humidity zone at an absolute humidity of notless than 0.5 g/m³ and less than 6.0 g/m³ and a high humidity zone at anabsolute humidity of not less than 7.0 g/m³ and less than 25.0 g/m³ toform a heat resistant resin membrane on the porous membrane A; and

Step (ii): immersing the composite membrane obtained in the step (i), inwhich the heat resistant resin membrane is laminated, in a coagulationbath to convert the heat resistant resin membrane into a porous membraneB, and washing and drying the porous membrane B to obtain a batteryseparator.

Effects of the Invention

In the present invention, the heat resistant resin layer and thepolypropylene porous membrane have excellent adhesion, and even when theheat resistant resin layer is laminated, the amount of air resistanceincrease (which hereinafter may be abbreviated as the amount of airresistance increase) is small compared to the air resistance of thepolypropylene porous membrane alone. Thus, the battery separator of thepresent invention has excellent heat resistance and, at the same time,excellent ion permeability. Further, the battery separator of thepresent invention is characterized by having excellent adhesion betweenthe polypropylene porous membrane, a substrate, and the porous membranecomprising the heat resistant resin layer and inorganic particles orcross-linked polymer particles (which hereinafter may be referred tosimply as adhesion of the porous membrane comprising the heat resistantresin layer, adhesion of the porous membrane B, or simply as adhesion),and excellent processability (low curling properties) in a batteryassembly process.

DESCRIPTION OF EMBODIMENTS

In the present invention, when laminating a heat resistant resin layeron a polypropylene porous membrane having a thickness of less than 10μm, an advanced processing technique was used to provide excellentadhesion of the heat resistant resin layer due to appropriately formedanchoring of the heat resistant resin layer without causing significantincrease in air resistance.

“Significant increase in air resistance” herein means that thedifference between an air resistance (X) of the porous membrane A thatserves as a substrate and an air resistance (Y) of the entire batteryseparator is more than 100 sec/100 cc Air.

The summary of the battery separator of the present invention will bedescribed, but the present invention is, of course, not limited to thisrepresentative example.

The battery separator of the present invention is a battery separatorcomprising a composite porous membrane comprising a porous membrane Aand a porous membrane B laminated thereon.

First, the porous membrane A used in the present invention will bedescribed.

The porous membrane A comprises at least one layer, and at least one ofits outermost layers comprises a polypropylene resin. Such a porousmembrane A can be prepared by the stretching pore-forming process or thephase separation method.

The phase separation method is a method comprising melt-blending, forexample, polypropylene with a solvent for film formation, extruding theresulting molten mixture through a die, cooling the extrudate to form agel-like product, stretching the gel-like product obtained in at leastone direction, and removing the solvent for film formation to obtain aporous membrane. The stretching pore-forming process is a method inwhich, for example, a lamellar structure in a sheeted film beforestretching is controlled by employing low temperature extrusion and ahigh draft ratio in melt extrusion of polypropylene, and the film isuniaxially stretched to cause cleavages at lamellar interfaces tothereby form voids (i.e., lamella stretching method). Further, a methodhas also been proposed comprising adding, for example, to polypropylenea large amount of inorganic particles or resin particles incompatiblewith polypropylene to form a sheet, and stretching the sheet to causecleavages at interfaces between the particles and the polypropyleneresin to thereby form voids. Another example is, for example, theβ-crystal method in which β-crystals with low crystal density (crystaldensity: 0.922 g/cm³) are formed when forming an unstretched sheet bymelt extrusion of polypropylene; the sheet is stretched to thereby causecrystal transition to α-crystals with high crystal density (0.936g/cm³); and pores are formed by the difference in crystal densitybetween the two. In this β-crystal method, to form a large number ofpores in a stretched film, it is necessary to selectively form a largeamount of β-crystals in an unstretched sheet before stretching.Therefore, in this β-crystal method, it is important to form β-crystalsunder specific melt crystallization conditions using a β-crystalnucleating agent. As a β-crystal nucleating agent, in addition toquinacridone compounds which have long been used, materials having aneven higher β-crystal forming ability have been proposed.

The porous membrane A may be a monolayer membrane or a multilayermembrane composed of two or more layers (e.g.,polypropylene/polyethylene/polypropylene) different in molecular weight,average pore size, or thermal properties. When the porous membrane A hasa layer structure of polypropylene/polyethylene/polypropylene, thethickness of one polypropylene layer is preferably 2.0 μm or more. Whenthe lower limit of the thickness of the polypropylene layer is in thispreferred range, sufficient mechanical strength is provided.

When the porous membrane A is a monolayer membrane, examples of theproduction method include the phase separation method and the stretchingpore-forming process described above. When the porous membrane A is amultilayer membrane of two or more layers, it is only required that atleast one surface layer be a polypropylene layer, and components of theother layer are not critical. When the porous membrane A is a multilayermembrane of two or more layers, it can be produced by a methodcomprising melt-blending each of the polyolefins constituting, forexample, an A1 layer and an A2 layer with a solvent for film formation,feeding the resulting molten mixtures from each extruder to one die tointegrate gel sheets constituting each component, and co-extruding theintegrated gel sheets, or a method comprising laminating gel sheetsconstituting each layer and heat-fusing the laminate. The co-extrusionmethod is preferred because a high interlayer adhesive strength iseasily achieved; high permeability is easily maintained becausecontinuous pores are easily formed between layers; and productivity ishigh.

Further, the polypropylene resin in the porous membrane A preferably hasa mass average molecular weight (Mw) of 300,000 or more, more preferably400,000 or more, and still more preferably 500,000 or more, in terms ofprocess workability and mechanical strengths to withstand variousexternal pressures caused when wound around an electrode, such astensile strength, elastic modulus, elongation, and pin puncturestrength. The upper limit of the Mw is preferably 4,000,000 or less,more preferably 3,000,000 or less. When the upper limit of the Mw of thepolypropylene resin in the porous membrane A is in this preferred range,excellent fluidity during melt extrusion is provided, and it is easy toform a sheet. The molecular weight distribution (Mw/Mn), the ratio of Mwto number average molecular weight (Mn), of polypropylene is notcritical, but is preferably 1.01 to 100, more preferably 1.1 to 50.

In the present invention, a porous membrane having a polypropylene resinlayer as an outermost layer is used as a substrate membrane, and thepolypropylene resin may contain other resins such as a polyethyleneresin as long as it is mainly composed of the polypropylene resin. Thepercentage of polypropylene is not less than 50% by weight in a resinmixture. When the percentage of polypropylene is in this preferredrange, the porous membrane A has excellent meltdown properties andelectrolyte solution retention. The percentage of polypropylene ispreferably not less than 75% by weight, more preferably not less than90% by weight. A copolymer of propylene and other olefins may also beused. The content of propylene units is not less than 50% by weight inthe copolymer. When the content of the propylene units is in thispreferred range, the porous membrane A has excellent meltdown propertiesand electrolyte solution retention. The percentage of polypropylene ismore preferably not less than 75% by weight, still more preferably notless than 90% by weight.

Preferred examples of comonomers copolymerized with propylene includeunsaturated hydrocarbons; for example, ethylene, and α-olefins such as1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, and 1-octene.

The pattern of copolymerization may be any pattern: alternating, random,block, or graft.

The porous membrane A needs to have a function (pore-blocking function)of blocking pores in the case of abnormal charge and discharge reaction.Accordingly, the melting point (softening point) of the constituentresin is preferably 70 to 150° C., more preferably 80 to 140° C., andstill more preferably 100 to 130° C. When the melting point (softeningpoint) of the resin constituting the porous membrane A is in thispreferred range, the pore-blocking function will not be activated innormal use, and, therefore, a battery will not be inoperable, while thepore-blocking function is activated before an abnormal reaction proceedsenough, and, therefore, sufficient safety can be ensured.

The porous membrane A used in the present invention has a thickness ofless than 10.0 μm. The upper limit is preferably 9.5 μm, more preferably9.0 μm. The lower limit is 5.0 μm, preferably 6.0 μm. When the thicknessis thinner than 5.0 μm, membrane strength and pore-blocking function ofpractical use may not be provided, and when it is not less than 10.0 μm,the area per unit volume of a battery case is significantly restricted,which is not suitable for increase in battery capacity which is expectedto progress in the future.

The upper limit of the air resistance of the porous membrane A ispreferably 500 sec/100 cc Air, more preferably 40 sec/100 cc Air, andstill more preferably 300 sec/100 cc Air; the lower limit is 50 sec/100cc Air, preferably 70 sec/100 cc Air, and more preferably 100 sec/100 ccAir.

The upper limit of the porosity of the porous membrane A is 70%,preferably 60%, and more preferably 55%. The lower limit is 30%,preferably 35%, and more preferably 40%. When the air resistance ishigher than 500 sec/100 cc Air or when the porosity is lower than 30%,sufficient charge and discharge properties, particularly, ionpermeability (charge and discharge operating voltage) of a battery andthe lifetime of a battery (closely related to the amount of electrolyticsolution retained) are not sufficient, and when these limits areexceeded, it is likely that functions of a battery cannot be fullyexerted. When the air resistance is lower than 50 sec/100 cc Air or whenthe porosity is higher than 70%, sufficient mechanical strength andinsulation properties cannot be provided, which increases thepossibility of a short circuit during charge and discharge.

The average pore size of the porous membrane A is 0.01 to 1.0 μm,preferably 0.05 to 0.5 μm, and more preferably 0.1 to 0.3 μm because ithas a great influence on pore-blocking speed. When the average pore sizeis smaller than 0.01 μm, it is difficult to produce the anchoring effectof a heat resistant resin, and thus sufficient adhesion of the heatresistant resin may not be provided; besides it is highly likely thatthe air resistance significantly deteriorates in complexation. When itis larger than 1.0 μm, phenomena can occur, such as slow response of apore-blocking phenomenon to temperature, shift of a pore-blockingtemperature depending on the temperature rise rate to the highertemperature side, and the like.

Further, for the surface condition of the porous membrane A, when it hasa surface roughness (arithmetic average roughness) of 0.01 to 0.5 μm,adhesion to the porous membrane B tends to be stronger. When the surfaceroughness (arithmetic average roughness) of the porous membrane A is inthis preferred range, adhesion to the porous membrane B is sufficientlystrong, and, in addition, decrease in mechanical strength of the porousmembrane A or transcription of irregularities to the surface of theporous membrane B will not occur.

In the present invention, the combination of a porous membrane Acomprising a resin having a glass transition temperature or meltingpoint of not higher than 150° C. and a porous membrane B comprising aresin having a glass transition temperature or melting point of higherthan 150° C. is preferred in order to have both a pore-blocking functionand a thermal-rupture-resistant function that are important particularlyfor a lithium ion battery separator.

The porous membrane B will now be described in more detail.

The porous membrane B comprises a heat resistant resin and inorganicparticles or cross-linked polymer particles. To serve to support andreinforce the porous membrane A with its heat resistance, the glasstransition temperature of the constituent resin is preferably 150° C. orhigher, more preferably 180° C. or higher, and still more preferably210° C. or higher, and it is not necessary to place an upper limit. Whenthe glass transition temperature is higher than a decompositiontemperature, it is preferred that the decomposition temperature be inthe above range. When the lower limit of the glass transitiontemperature or melting point of the resin constituting the porousmembrane B is in the preferred range above, a sufficientthermal-rupture-resistant temperature can be achieved, and high safetycan be ensured.

The heat resistant resin constituting the porous membrane B may be anyresin as long as it has excellent heat resistance.

For example, a resin mainly composed of polyamide-imide, polyimide,polyamide, or fluororesin can be suitably used, and these resins may beused alone or in combination with other materials.

A polyamide-imide resin will be described in detail below as a firstexample of the heat resistant resin.

In general, a polyamide-imide resin is synthesized by a common methodsuch as the acid chloride method using trimellitic acid chloride anddiamine or the diisocyanate method using trimellitic acid anhydride anddiisocyanate, and the diisocyanate method is preferred in terms ofproduction cost.

Examples of the acid component used in the synthesis of apolyamide-imide resin include trimellitic acid anhydride (chloride), aportion of which can be replaced with other polybasic acid or anhydridethereof. Examples thereof include tetracarboxylic acids such aspyromellitic acid, biphenyltetracarboxylic acid,biphenylsulfonetetracarboxylic acid, benzophenonetetracarboxylic acid,biphenyl ether tetracarboxylic acid, ethylene glycol bistrimellitate,and propylene glycol bistrimellitate, and anhydrides thereof; aliphaticdicarboxylic acids such as oxalic acid, adipic acid, malonic acid,sebacic acid, azelaic acid, dodecane dicarboxylic acid,dicarboxypolybutadiene, dicarboxypoly(acrylonitrile-butadiene), anddicarboxypoly(styrene-butadiene); alicyclic dicarboxylic acids such as1,4-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid,4,4′-dicyclohexylmethanedicarboxylic acid, and dimer acid; and aromaticdicarboxylic acids such as terephthalic acid, isophthalic acid,diphenylsulfonedicarboxylic acid, diphenyl ether dicarboxylic acid, andnaphthalenedicarboxylic acid. Among them, 1,3-cyclohexanedicarboxylicacid and 1,4-cyclohexanedicarboxylic acid are preferred in terms ofelectrolyte resistance. In terms of shutdown properties, dimer acid, anddicarboxypolybutadiene, dicarboxypoly(acrylonitrilebutadiene), anddicarboxypoly(styrene-butadiene) with a molecular weight of 1,000 ormore are preferred.

Also, a portion of a trimellitic acid compound can be replaced with aglycol to introduce a urethane group into a molecule. Examples ofglycols include alkylene glycols such as ethylene glycol, propyleneglycol, tetramethylene glycol, neopentyl glycol, and hexanediol;polyalkylene glycols such as polyethylene glycol, polypropylene glycol,and polytetramethylene glycol; and polyesters with terminal hydroxylgroups synthesized from one or more of the dicarboxylic acids describedabove and one or more of the glycols described above, among whichpolyethylene glycol and polyesters with terminal hydroxyl groups arepreferred in terms of a shutdown effect. The number average molecularweight of them is preferably 500 or more, more preferably 1,000 or more.The upper limit is preferably less than 8,000, but is not limitedthereto.

When a portion of the acid component is replaced with at least one fromthe group consisting of dimer acid, polyalkylene ether, polyester, andbutadiene rubber containing any one of a carboxyl group, a hydroxylgroup, and an amino group at its terminal, it is preferable to replace 1to 60 mol % of the acid component.

The diamine (diisocyanate) component used in the synthesis of apolyamide-imide resin is preferably composed of o-tolidine andtolylenediamine, and examples of the component that substitutes for aportion thereof include aliphatic diamines such as ethylenediamine,propylenediamine, and hexamethylenediamine, and diisocyanates thereof;alicyclic diamines such as 1,4-cyclohexanediamine,1,3-cyclohexanediamine, and dicyclohexylmethanediamine, anddiisocyanates thereof; and aromatic diamines such as m-phenylenediamine,p-phenylenediamine, 4,4′-diaminodiphenylmethane, 4,4′-diaminodiphenylether, 4,4′-diaminodiphenylsulfone, benzidine, xylylenediamine, andnaphthalenediamine, and diisocyanates thereof, among whichdicyclohexylmethanediamine and a diisocyanate thereof are more preferredin terms of reactivity, cost, and electrolyte resistance, and4,4′-diaminodiphenylmethane, naphthalenediamine, and diisocyanatesthereof are still more preferred. In particular, o-tolidine diisocyanate(TODI), 2,4-tolylene diisocyanate (TDI), and a blend thereof are mostpreferred. In order particularly to improve adhesion of the heatresistant porous membrane B, o-tolidine diisocyanate (TODI) which hashigh stiffness accounts for 50 mol % or more, preferably 60 mol % ormore, and more preferably 70 mol % or more of total isocyanates.

A polyamide-imide resin can be readily prepared by stirring in a polarsolvent such as N,N′-dimethylformamide, N,N′-dimethylacetamide,N-methyl-2-pyrrolidone, or γ-butyrolactone with heating at 60 to 200° C.In this case, an amine such as triethylamine or diethylenetriamine; analkali metal salt such as sodium fluoride, potassium fluoride, cesiumfluoride, or sodium methoxide; or the like can also be used as acatalyst as required.

When a polyamide-imide resin is used in the present invention, thepolyamide-imide resin preferably has a logarithmic viscosity of 0.5 dl/gor more. When the lower limit of the logarithmic viscosity of thepolyamide-imide resin is in this preferred range, sufficient meltdownproperties are provided. Further, the porous membrane cannot be brittle,and a sufficient anchoring effect is produced, which leads to excellentadhesion. The upper limit is preferably lower than 2.0 dl/g in view ofprocessability and solvent solubility.

A fluororesin will now be described in detail as a second example of theheat resistant resin.

As a fluororesin, it is preferable to use at least one selected from thegroup consisting of vinylidene fluoride homopolymer, vinylidenefluoride/fluorinated olefin copolymer, vinyl fluoride homopolymer, andvinyl fluoride/fluorinated olefin copolymer. Polytetrafluoroethylene isparticularly preferred. These polymers have high affinity for nonaqueouselectrolyte solution, proper heat resistance, and high chemical andphysical stability to nonaqueous electrolyte solution, and therefore canmaintain an affinity for electrolyte solution sufficiently even whenused at a high temperature.

The porous membrane B is obtained by applying to a given substrate filma fluororesin solution (varnish) obtained by dissolution in a solventthat is able to dissolve a fluororesin and miscible with water, causingphase separation between the fluororesin and the solvent miscible withwater under humidified conditions, and further coagulating thefluororesin by injection into a water bath (coagulation bath). A phaseseparation aid may optionally be added to the varnish.

Examples of solvents that can be used to dissolve the fluororesininclude N,N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP),hexamethylphosphoric triamide (HMPA), N,N-dimethylformamide (DMF),dimethyl sulfoxide (DMSO), γ-butyrolactone, chloroform,tetrachloroethane, dichloroethane, 3-chloronaphthalene,parachlorophenol, tetralin, and acetonitrile, and the solvent can bearbitrarily selected depending on the solubility of resins.

The porous membrane B of the present invention is obtained by applyingto a given substrate film a heat resistant resin solution (whichhereinafter may be referred to as varnish) obtained by dissolution in asolvent that is able to dissolve a heat resistant resin and misciblewith water, causing phase separation between the heat resistant resinand the solvent miscible with water under humidified conditions, andfurther coagulating the heat resistant resin by injection into a waterbath (the water bath hereinafter may be referred to as a coagulationbath). A phase separation aid may optionally be added to the varnish.

Examples of solvents that can be used to dissolve the heat resistantresin include N,N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone(NMP), hexamethylphosphoric triamide (HMPA), N,N-dimethylformamide(DMF), dimethyl sulfoxide (DMSO), γ-butyrolactone, chloroform,tetrachloroethane, dichloroethane, 3-chloronaphthalene,parachlorophenol, tetralin, acetone, and acetonitrile, and the solventcan be arbitrarily selected depending on the solubility of resins.

The porous membrane B serves to support and reinforce the porousmembrane A with its heat resistance. Thus, the melting point of thefluororesin constituting the porous membrane B is preferably 150° C. orhigher, more preferably 180° C. or higher, and still more preferably210° C. or higher, and the upper limit is not particularly limited. Whenthe melting point is higher than a decomposition temperature, it ispreferred that the decomposition temperature be in the above range. Whenthe lower limit of the glass transition temperature or melting point ofthe fluororesin is in the preferred range above, a sufficientthermal-rupture-resistant temperature can be achieved, and high safetycan be ensured.

The solids concentration of the varnish is not critical as long as thevarnish can be applied uniformly, but is preferably 10% by weight to 50%by weight, more preferably 20% by weight to 45% by weight. When thesolids concentration of the varnish is in this preferred range, theresulting porous membrane B cannot be brittle, and sufficient adhesionto the porous membrane B is provided.

The phase separation aid used in the present invention is at least oneselected from water, alkylene glycols such as ethylene glycol, propyleneglycol, tetramethylene glycol, neopentyl glycol, and hexanediol,polyalkylene glycols such as polyethylene glycol, polypropylene glycol,and polytetramethylene glycol, water-soluble polyesters, water-solublepolyurethanes, polyvinyl alcohols, carboxymethylcellulose, and the like.The phase separation aid is preferably added in an amount in the rangeof 10 to 90 wt %, more preferably 20 to 80 wt %, and still morepreferably 30 to 70%, based on the solution weight of the varnish.

By mixing such a phase separation aid with the varnish, air resistance,surface porosity, and rate of formation of layer structure can be mainlycontrolled. When the amount added is less than the range describedabove, significant increase in phase separation rate may not beachieved. When the amount added is more than the range described above,a coating solution may become cloudy at the mixing stage, resulting inprecipitation of the resin component.

It is important to add inorganic particles or cross-linked polymerparticles to the varnish in order to reduce curling. Further, addinginorganic particles or cross-linked polymer particles to the varnishproduces effects of preventing internal short circuit due to the growthof dendrites on an electrode inside a battery (dendrite-preventingeffect), reducing the heat shrinkage rate, providing slipcharacteristics, and the like. The upper limit of the amount of suchparticles is preferably 98% by weight, more preferably 95% by weight.The lower limit is preferably 80% by weight, more preferably 85% byweight. When the amount of inorganic particles or cross-linked polymerparticles added to the varnish is in this preferred range, a sufficientcurling-reducing effect is produced; at the same time, the percentage ofthe heat resistant resin relative to the total volume of the porousmembrane B is sufficient, and the resin penetrates deep into pores ofthe porous membrane A, resulting in sufficient adhesion to the heatresistant resin layer.

Examples of the inorganic particles include calcium carbonate, calciumphosphate, amorphous silica, crystalline glass filler, kaolin, talc,titanium dioxide, alumina, silica-alumina composite oxide particles,barium sulfate, calcium fluoride, lithium fluoride, zeolite, molybdenumsulfide, and mica.

Examples of heat resistant cross-linked polymer particles includecross-linked polystyrene particles, cross-linked acrylic resinparticles, and cross-linked methyl methacrylate particles.

The average diameter of such particles is preferably 1.5 times to 50times, more preferably 2.0 times to 20 times the average pore size ofthe polypropylene porous membrane A.

When the ratio of the average diameter of such particles to the averagepore size of the polypropylene porous membrane A is in the preferredrange above, the heat resistant resin and the particles cannot block thepores of the polypropylene porous membrane A in a mixed state, and,therefore, significant increase in air resistance is prevented; at thesame time, the particles are unlikely to fall off during a batteryassembly process, and, therefore, serious defects in a battery can beeffectively prevented from occurring.

The thickness of the porous membrane B is preferably 1.0 to 5.0 μm, morepreferably 1.0 to 4.0 μm, and still more preferably 1.0 to 3.0 μm. Whenthe thickness of the porous membrane B is in this preferred range,membrane strength and insulation properties can be ensured when theporous membrane A melts and shrinks at or higher than its melting point.At the same time, the moderate percentage of the porous membrane Aprovides a sufficient pore-blocking function, and an abnormal reactioncan be prevented; the size when taken up will not be too large, which issuitable for increase in battery capacity which is expected to progressin the future; and further, curling is unlikely to increase, whichcontributes to improved productivity in a battery assembly process.

The porosity of the porous membrane B is preferably 30 to 90%, morepreferably 40 to 70%. When the porosity of the porous membrane B is inthis preferred range, the electrical resistance of the membrane cannotbe too high, and it is easy to apply a high current; at the same time,the membrane strength can be maintained at a high level. The airresistance of the porous membrane B, as measured by a method inaccordance with JIS P 8117, is preferably 1 to 600 sec/100 cc Air, morepreferably 50 to 500 sec/100 cc Air, and still more preferably 100 to400 sec/100 cc Air. When the air resistance of the porous membrane B isin this preferred range, high membrane strength is provided, and at thesame time, cycle characteristics can be maintained at a satisfactorylevel.

The upper limit of the total thickness of a battery separator obtainedby laminating the porous membrane B is 13 μm, more preferably 12 μm. Thelower limit is preferably not less than 5.0 μm, more preferably not lessthan 7.0 μm. When the total thickness of a battery separator obtained bylaminating the porous membrane B is in this preferred range, sufficientmechanical strength and insulation properties can be easily ensured; atthe same time, the amount of air resistance increase cannot be large,and the area of electrodes that can be loaded into a container can besufficiently ensured, which results in avoidance of decrease incapacity.

The battery separator of the present invention satisfies the relation ofthe difference (Y−X) between an air resistance X (sec/100 cc Air) of theporous membrane A and an air resistance Y (sec/100 cc Air) of the entirebattery separator: 20 sec/100 cc Air≦Y−X≦100 sec/100 cc Air. When Y−X isless than 20 sec/100 cc Air, sufficient adhesion of the heat resistantresin layer cannot be provided. When it is more than 100 sec/100 cc Air,significant increase in air resistance occurs, and, as a result, ionpermeability decreases when introduced into a battery, resulting in aseparator unsuitable for a high-performance battery.

Further, air resistance of the battery separator, which is one of themost important properties, is preferably 50 to 600 sec/100 cc Air, morepreferably 100 to 500 sec/100 cc Air, and still more preferably 100 to400 sec/100 cc Air. When the air resistance is in this preferred range,sufficient insulation properties are provided, and clogging of foreignsubstances, short circuit, and membrane rupture do not readily occur; atthe same time, the membrane resistance is not too high, and charge anddischarge properties and lifetime properties in a practical range areprovided.

In the present invention, the peeling strength F (A/B) at the interfacebetween the porous membrane A and the porous membrane B needs to satisfyF (A/B)≧1.0 N/25 mm. “Excellent adhesion” as used herein means that F(A/B) is 1.0 N/25 mm or more, and it is preferably 1.5 N/25 mm or more,and still more preferably 2.0 N/25 mm or more. F (A/B) means adhesion ofthe porous membrane B to the porous membrane A, and when it is less than1.0 N/25 mm, the heat resistant resin layer may be peeled off duringhigh-speed processing in the battery assembly process described above.

The process for producing the battery separator of the present inventionwill now be described.

The process for producing the battery separator of the present inventioncomprises the following steps (i) and (ii).

Step (i): applying a varnish comprising a heat resistant resin andinorganic particles or cross-linked polymer particles to a porousmembrane A comprising a polypropylene resin, and then passing the porousmembrane A through a low humidity zone at an absolute humidity of notless than 0.5 g/m³ and less than 6.0 g/m³ and a high humidity zone at anabsolute humidity of not less than 7.0 g/m³ and less than 25.0 g/m³ toform a heat resistant resin membrane on the porous membrane A; and

Step (ii): immersing the composite membrane obtained in the step (i), inwhich the heat resistant resin membrane is laminated, in a coagulationbath to convert the heat resistant resin membrane into a porous membraneB, and washing and drying the porous membrane B to obtain a batteryseparator.

A description will be given in more detail.

The porous membrane B is obtained by laminating a varnish mainlycomposed of a heat resistant resin solution, which is obtained bydissolution in a solvent that is able to dissolve a heat resistant resinand miscible with water, and the particles described above on a porousmembrane A comprising a given polypropylene resin using a coating methodto thereby form a heat resistant resin membrane, placing in a certainhumidity environment before or after the lamination to cause phaseseparation between the heat resistant resin and the solvent misciblewith water, and pouring into a water bath (coagulation bath) tocoagulate the heat resistant resin membrane. The varnish may be applieddirectly to a porous membrane A, or a method (transcription method) maybe used comprising applying the varnish once to a substrate film (e.g.,polypropylene film or polyester film), placing the coated film in acertain humidity environment to cause phase separation between the heatresistant resin component and the solvent component, and thentranscribing the porous membrane B onto the porous membrane A to achievelamination.

“Low humidity zone” as used herein refers to a zone where absolutehumidity is controlled to be less than 6.0 g/m³, the upper limit to bepreferably 4.0 g/m³, more preferably 3.0 g/m³, and the lower limit to be0.5 g/m³, preferably 0.8 g/m³.

When the absolute humidity is less than 0.5 g/m³, phase separation doesnot proceed sufficiently, and thus a porous membrane is less likely tobe formed, which can lead to a large amount of air resistance increase.When the absolute humidity is not less than 6 g/m³, the resinconstituting the porous membrane B starts to coagulate in parallel withthe phase separation, and the resin component constituting the porousmembrane B does not infiltrate into the porous membrane A sufficiently;consequently, sufficient adhesion of the heat resistant resin is notprovided. Further, when the time of passage through the low humidityzone is less than 3 seconds, the phase separation does not proceedsufficiently, and when it is more than 20 seconds, coagulation of theresin constituting the porous membrane B proceeds; which are notpreferred.

The coated film is then passed through the high humidity zone over 3seconds to 10 seconds.

“High humidity zone” as used herein refers to a zone where the lowerlimit of absolute humidity is controlled to be 7.0 g/m³, preferably 8.0g/m³, and the upper limit to be 25 g/m³, preferably 17 g/m³, and morepreferably 15 g/m³. When the absolute humidity is less than 7.0 g/m³,gelation (defluidization) does not proceed sufficiently, and thereforeinfiltration of the resin component constituting the porous membrane Binto the porous membrane A proceeds too far, which leads to a largeamount of air resistance increase. When the absolute humidity is morethan 25 g/m³, coagulation of the resin component constituting the porousmembrane B proceeds too far, and infiltration of the resin componentconstituting the porous membrane B into the porous membrane A is toolittle; consequently, sufficient adhesion may not be provided.

For both the low humidity zone and the high humidity zone, temperatureconditions are not critical as long as the absolute humidity is in theranges described above, but a preferred range is 20° C. to 50° C. fromthe standpoint of energy saving.

Examples of the method of applying the varnish include reverse rollcoating, gravure coating, kiss coating, roll brushing, spray coating,air knife coating, meyer bar coating, pipe doctor method, blade coating,and die coating, and these methods can be used alone or in combination.

In the coagulation bath, the resin component and the particles coagulateinto three-dimensional network. The immersion time in the coagulationbath is preferably not less than 3 seconds. If it is less than 3seconds, coagulation of the resin component may not proceedsufficiently. Although the upper limit is not limited, 10 seconds isenough.

Further, the unwashed porous membrane described above is immersed in anaqueous solution containing a good solvent for resin constituting theporous membrane B in an amount of preferably 1 to 20% by weight, morepreferably 5 to 15% by weight, and the washing step using pure water andthe drying step using hot air at 100° C. or lower are conducted, wherebya final battery separator can be obtained.

For the washing in wet film formation, common methods such as warming,ultrasonic irradiation, and bubbling can be used. Further, for keepingthe concentration in each bath constant to increase washing efficiency,removing the solution in the porous membrane between the baths iseffective. Specific examples include extruding the solution in theporous layer with air or inert gas, squeezing out the solution in themembrane physically with a guide roll, and the like.

According to the method described above, even when the porous membrane Ahas a thickness of less than 10 μm, a balance between adhesion and airresistance is provided, and a battery separator with a small variationin air resistance can be obtained. In the present invention, the airresistance variation is preferably 50 sec/100 cc Air or less, morepreferably 40 sec/100 cc Air or less, and still more preferably 30sec/100 cc Air or less.

For the washing in wet film formation, common methods such as warming,ultrasonic irradiation, and bubbling can be used. Further, for keepingthe concentration in each bath constant to increase washing efficiency,removing the solution in the porous membrane between the baths iseffective. Specific examples include extruding the solution in theporous layer with air or inert gas, squeezing out the solution in themembrane physically with a guide roll, and the like.

The battery separator of the present invention is desirably stored dry,but when it is difficult to store it absolutely dry, it is preferable toperform a vacuum drying treatment at 100° C. or lower immediately beforeuse.

The battery separator of the present invention can be used as aseparator, for example, for secondary batteries such as anickel-hydrogen battery, a nickel-cadmium battery, a nickel-zincbattery, a silver-zinc battery, a lithium ion secondary battery, and alithium polymer secondary battery, plastic film capacitors, ceramiccapacitors, and electric double layer capacitors. In particular, thebattery separator of the present invention is preferably used as alithium ion secondary battery separator. A description will be givenbelow with reference to a lithium ion secondary battery.

In a lithium ion secondary battery, a cathode and an anode are laminatedwith a separator interposed therebetween, and the separator contains anelectrolytic solution (electrolyte). Structure of the electrodes is notcritical and may be a known structure. For example, the structure can bean electrode structure in which a cathode and an anode in the form of adisk are arranged opposed to each other (coin-type), an electrodestructure in which a cathode and an anode in the form of a flat plateare alternately laminated (laminated-type), an electrode structure inwhich a cathode and an anode in the form of a strip are laminated andwound (wound-type), and the like.

The cathode typically has a current collector and a cathode activematerial layer that is formed on the surface of the current collectorand contains a cathode active material capable of occluding andreleasing lithium ions. Examples of cathode active materials includeinorganic compounds such as transition metal oxides, composite oxides oflithium and a transition metal (lithium composite oxides), andtransition metal sulfides, and examples of transition metals include V,Mn, Fe, Co, and Ni. Preferred examples of lithium composite oxides amongthe cathode active materials include lithium nickel oxide, lithiumcobalt oxide, lithium manganese oxide, and layered lithium compositeoxides based on α-NaFeO₂ structure.

The anode has a current collector and an anode active material layerthat is formed on the surface of the current collector and contains ananode active material. Examples of anode active materials includecarbonaceous materials such as natural graphite, artificial graphite,cokes, and carbon black. The electrolytic solution can be obtained bydissolving a lithium salt in an organic solvent. Examples of lithiumsalts include LiClO₄, LiPF₆, LiAsF₆, LiSbF₆, LiBF₄, LiCF₃SO₃,LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃, Li₂B₁₀Cl₁₀, LiN(C₂F₅SO₂)₂, LiPF₄(CF₃)₂,LiPF₃(C₂F₅)₃, lower aliphatic carboxylic acid lithium salts, andLiAlCl₄. These may be used alone or in combination of two or morethereof. Examples of organic solvents include high-boiling andhigh-dielectric organic solvents such as ethylene carbonate, propylenecarbonate, ethyl methyl carbonate, and γ-butyrolactone; and low-boilingand low-viscosity organic solvents such as tetrahydrofuran,2-methyltetrahydrofuran, dimethoxyethane, dioxolane, dimethyl carbonate,and diethyl carbonate. These may be used alone or in combination of twoor more thereof. In particular, high-dielectric organic solvents havehigh viscosity, and low-viscosity organic solvents have a low dielectricconstant; therefore, it is preferable to use the two in combination.

When assembling a battery, the separator of the present invention isimpregnated with an electrolytic solution. This provides the separatorwith ion permeability. In general, the impregnation treatment isperformed by immersing a microporous porous membrane in an electrolyticsolution at normal temperature. For example, in the case of assembling acylindrical battery, a cathode sheet, a separator (composite porousmembrane), and an anode sheet are first laminated in the ordermentioned, and this laminate is taken up from one end to provide awound-type electrode element. This electrode element is then insertedinto a battery can and impregnated with the electrolytic solutiondescribed above, and, further, a battery lid that is provided with asafety valve and serves also as a cathode terminal is caulked via agasket to thereby obtain a battery.

Examples

The present invention will now be described in detail by way of example,but the present invention is not limited to the examples. Themeasurements in the examples are values determined by the followingmethods.

1. Thickness

A thickness was measured using a contact thickness meter (digitalmicrometer M-30 manufactured by Sony Manufacturing Systems Corporation).

2. Adhesion of Porous Membrane B

Adhesive tape (available from NICHIBAN CO., LTD., No. 405; 24 mm wide)was applied to the porous membrane B surface of a separator obtained inExamples and Comparative Examples, and the separator was cut to a widthof 24 mm and a length of 150 mm to prepare a test sample.

A peeling strength at the interface between a porous membrane A and aporous membrane B was measured by the peeling method (peel rate: 500mm/min, T-peel) under the conditions of 23° C. and 50% RH using atensile tester (“Tensilon RTM-100” manufactured by A & D Company,Limited). Measurements were made over time within 100 mm from the startto the end of the measurements, and an average value of the measurementswas calculated and converted to a value per 25 mm width, which was usedas a peeling strength. At the peeled interface described above, theporous membrane B surface can remain on the porous membrane A side, butalso in this case a value was calculated as a peeling strength at theinterface between the porous membrane A and the porous membrane B.

3. Average Pore Size

The average pore size of a porous membrane A was measured by thefollowing method.

A test piece was fixed onto a cell for measurement using double-sidedtape; platinum or gold was vacuum-deposited for several minutes; andmeasurements were made at an appropriate magnification.

Arbitrary 10 points on an image obtained by SEM measurement wereselected, and an average value of pore sizes at the 10 points was usedas an average pore size of the test piece.

4. Air Resistance

Using a Gurley densometer type B manufactured by TESTER SANGYO CO.,LTD., a battery separator was fixed between a clamping plate and anadapter plate such that wrinkling did not occur, and an air resistancewas measured according to JIS P-8117. The sample was 10-cm square, andmeasuring points were the center and four corners, five points in total,of the sample; the average value was used as an air resistance (sec/100cc Air).

When the length of a side of the sample is less than 10 cm, a valueobtained by measuring air resistance at five points at intervals of 5 cmmay be used. Air resistance variation (sec/100 cc Air) was determinedfrom the difference between the maximum value and the minimum value ofthe five measurements.

5. Logarithmic Viscosity

A solution obtained by dissolving 0.5 g of a heat resistant resin in 100ml of NMP was measured at 25° C. using an Ubbelohde viscosity tube.

6. Melting Point

Using a differential scanning calorimeter (DSC) DSC 6220 manufactured bySII NanoTechnology Inc., 5 mg of a resin sample was placed in a nitrogengas atmosphere, and the temperature was raised at a rate of 20° C./min.The peak temperature of melting peaks observed was used as a meltingpoint.

7. Glass Transition Temperature

A resin solution or a resin solution obtained by dipping a batteryseparator in a good solvent to dissolve only a heat resistant resinlayer was applied at an appropriate gap using an applicator to a PETfilm (E5001 available from TOYOBO CO., LTD.) or a polypropylene film(PYLEN-OT (registered trademark) available from TOYOBO CO., LTD.),predried at 120° C. for 10 minutes, and then peeled. The film obtainedwas fixed to a metal frame of an appropriate size with heat resistantadhesive tape, and, in such a state, further dried under vacuum at 200°C. for 12 hours to obtain a dry film. A test piece 4 mm wide×21 mm longwas cut out from the dry film obtained, and using a dynamicviscoelasticity measuring apparatus (DVA-220 manufactured by IT KeisokuSeigyo Co., Ltd.) at a measuring length of 15 mm, a storage elasticmodulus (E′) was measured in the range from room temperature to 450° C.under the conditions of 110 Hz and a temperature rise rate of 4° C./min.At an inflection point of the storage elastic modulus (E′) at this time,the temperature at the intersection of an extended baseline at or lowerthan a glass transition temperature and a tangent line showing a maximumslope at or higher than the inflection point was used as a glasstransition temperature.

8. Porosity

A 10-cm square sample was provided, and its sample volume (cm³) and mass(g) were measured; a porosity (%) was calculated from the resultsobtained using the following equation.

Porosity=(1−mass/(resin density×sample volume))×100

9. Evaluation of Curling Properties (Warpage)

A battery separator obtained in Examples and Comparative Examples wascut to a size of 100 mm wide×300 mm long. Static electricity was removedthoroughly with an antistatic brush, and then the sample was placed on ahorizontally-disposed glass plate with the porous membrane B up. Bothwidthwise edges were then fixed by 10 mm, and a lift height at bothlengthwise edges was measured. The average value was determined.

Example A-1 Synthesis of Heat Resistant Resin

Into a four-necked flask equipped with a thermometer, a cooling tube,and a nitrogen gas introduction tube, 1 mol of trimellitic acidanhydride (TMA), 0.8 mol of o-tolidine diisocyanate (TODI), 0.2 mol of2,4-tolylene diisocyanate (TDI), and 0.01 mol of potassium fluoride wereloaded together with N-methyl-2-pyrrolidone to a solids concentration of20% and stirred at 100° C. for 5 hours, and then the resulting mixturewas diluted with N-methyl-2-pyrrolidone to a solids concentration of 14%to synthesize a polyamide-imide resin solution (PI-a). Thepolyamide-imide resin obtained had a logarithmic viscosity of 1.35 dl/gand a glass transition temperature of 320° C.

The polyamide-imide resin solution (PI-a), alumina particles having anaverage diameter of 0.50 μm, and N-methyl-2-pyrrolidone were mixed at aweight ratio of 26:34:40, and the resulting mixture was placed into apolypropylene container together with zirconium oxide beads (trade name:“Torayceram” (registered trademark) beads available from TORAYINDUSTRIES, INC., diameter: 0.5 mm) and dispersed for 6 hours using apaint shaker (manufactured by Toyo Seiki Seisaku-Sho, Ltd.). Thedispersion was then filtered through a filter with a filtration limit of5 μm to prepare a varnish.

The varnish was applied to a porous membrane A (polypropylene,thickness: 9.0 μm, porosity: 40%, average pore size: 0.10 μm, and airresistance: 450 sec/100 cc Air) by blade coating. The coated membranewas passed through a low humidity zone at a temperature of 25° C. and anabsolute humidity of 1.8 g/m³ over 8 seconds, and then through a highhumidity zone at a temperature of 25° C. and an absolute humidity of12.0 g/m³ over 5 seconds, immersed in an aqueous solution containing 5%by weight of N-methyl-2-pyrrolidone for 10 seconds, washed with purewater, and then passed through a hot-air drying furnace at 70° C. fordrying to obtain a battery separator having a final thickness of 11.5μm.

Example A-2

A battery separator was obtained in the same manner as in Example A-1except that the absolute humidity in the low humidity zone was 4.0 g/m³.

Example A-3

A battery separator was obtained in the same manner as in Example A-1except that the absolute humidity in the low humidity zone was 5.5 g/m³.

Example A-4

A battery separator was obtained in the same manner as in Example A-3except that the absolute humidity in the high humidity zone was 7.0g/m³.

Example A-5

A battery separator was obtained in the same manner as in Example A-3except that the absolute humidity in the high humidity zone was 16.0g/m³.

Example A-6

A battery separator was obtained in the same manner as in Example A-3except that the mixing ratio of the polyamide-imide resin solution(PI-a), alumina particles having an average diameter of 0.50 μm, andN-methyl-2-pyrrolidone was 26:15:59 (weight ratio).

Example A-7

A battery separator was obtained in the same manner as in Example A-3except that the mixing ratio of the polyamide-imide resin solution(PI-a), alumina particles having an average diameter of 0.50 μm, andN-methyl-2-pyrrolidone was 15:41:44 (weight ratio).

Example A-8

A battery separator was obtained in the same manner as in Example A-3except that a polypropylene porous film having a thickness of 9.5 μm, aporosity of 40%, an average pore size of 0.15 μm, and an air resistanceof 300 sec/100 cc Air was used as a porous film A.

Example A-9

A battery separator was obtained in the same manner as in Example A-3except that a polypropylene porous film having a thickness of 7.0 μm, aporosity of 40%, an average pore size of 0.15 μm, and an air resistanceof 220 sec/100 cc Air was used as a porous film A.

Example A-10

A battery separator was obtained in the same manner as in Example A-3except that a polypropylene porous film having a thickness of 5.0 μm, aporosity of 40%, an average pore size of 0.15 μm, and an air resistanceof 200 sec/100 cc Air was used as a porous film A.

Example A-11

A battery separator was obtained in the same manner as in Example A-3except that the amount of the varnish was adjusted to a final thicknessof 10.5 μm.

Example A-12

Into a four-necked flask equipped with a thermometer, a cooling tube,and a nitrogen gas introduction tube, 1 mol of trimellitic acidanhydride (TMA), 0.80 mol of o-tolidine diisocyanate (TODI), 0.20 mol ofdiphenylmethane-4,4′-diisocyanate (MDI), and 0.01 mol of potassiumfluoride were loaded together with N-methyl-2-pyrrolidone to a solidsconcentration of 20% and stirred at 100° C. for 5 hours, and then theresulting mixture was diluted with N-methyl-2-pyrrolidone to a solidsconcentration of 14% to synthesize a polyamide-imide resin solution (b).The polyamide-imide resin obtained had a logarithmic viscosity of 1.05dl/g and a glass transition temperature of 313° C.

A battery separator was obtained in the same manner as in Example A-3except that a varnish comprising the polyamide-imide resin solution(PI-b) in place of the polyamide-imide resin solution (PI-a) was used.

Example A-13

Into a four-necked flask equipped with a thermometer, a cooling tube,and a nitrogen gas introduction tube, 1 mol of trimellitic acidanhydride (TMA), 0.60 mol of o-tolidine diisocyanate (TODI), 0.40 mol ofdiphenylmethane-4,4′-diisocyanate (MDI), and 0.01 mol of potassiumfluoride were loaded together with N-methyl-2-pyrrolidone to a solidsconcentration of 20% and stirred at 100° C. for 5 hours, and then theresulting mixture was diluted with N-methyl-2-pyrrolidone to a solidsconcentration of 14% to synthesize a polyamide-imide resin solution (c).The polyamide-imide resin obtained had a logarithmic viscosity of 0.85dl/g and a glass transition temperature of 308° C.

A battery separator was obtained in the same manner as in Example A-3except that a varnish comprising the polyamide-imide resin solution(PI-c) in place of the polyamide-imide resin solution (PI-a) was used.

Example A-14

A battery separator was obtained in the same manner as in Example A-3except that a varnish comprising titanium oxide particles (availablefrom Titan Kogyo, Ltd., trade name: KR-380, average particle size: 0.38μm) in place of alumina particles was used.

Example A-15

A battery separator was obtained in the same manner as in Example A-3except that a polypropylene porous film having a thickness of 9.0 μm, aporosity of 38%, an average pore size of 0.15 μm, and an air resistanceof 130 sec/100 cc Air was used as a porous film A.

Example A-16

A battery separator was obtained in the same manner as in Example A-3except that a varnish comprising cross-linked polymer particles(polymethyl methacrylate cross-linked particles (product name: “Epostar”(registered trademark) MA, type 1002, available from NIPPON SHOKUBAICO., LTD., average particle size: 2.50 μm)) in place of aluminaparticles was used.

Example A-17

The same varnish as in Example A-1 was applied to a corona-treatedsurface of a polyethylene terephthalate resin film having a thickness of50 μm (E5101 available from TOYOBO CO., LTD.) by blade coating. Thecoated film was passed through a low humidity zone at a temperature of25° C. and an absolute humidity of 1.8 g/m³ over 8 seconds, and thenthrough a high humidity zone at a temperature of 25° C. and an absolutehumidity of 12 g/m³ over 5 seconds. After 1.7 seconds, a porous film A(polypropylene, thickness: 9.0 μm, porosity: 45%, average pore size:0.15 μm, and air resistance: 450 sec/100 cc Air) was laminated on thegel-like heat resistant resin surface described above, and the laminatewas placed into an aqueous solution containing 5% by weight ofN-methyl-2-pyrrolidone, washed with pure water, and then passed througha hot-air drying furnace at 70° C. for drying to obtain a batteryseparator having a final thickness of 11.5 μm.

Example A-18

A battery separator was obtained in the same manner as in Example A-3except that a porous membrane having a three-layer structure ofpolypropylene/polyethylene/polypropylene (thickness: 9.0 μm (3.0 μm/3.0μm/3.0 μm), porosity: 40%, average pore size: 0.10 μm, and airresistance: 400 sec/100 cc Air) was used as a porous film A.

Comparative Example A-1

A battery separator was obtained in the same manner as in Example A-3except that the absolute humidity in the low humidity zone was 7.0 g/m³.

Comparative Example A-2

A battery separator was obtained in the same manner as in Example A-3except that the absolute humidity in the high humidity zone was 5.0g/m³.

Comparative Example A-3

Into a four-necked flask equipped with a thermometer, a cooling tube,and a nitrogen gas introduction tube, 1 mol of trimellitic acidanhydride (TMA), 0.76 mol of o-tolidine diisocyanate (TODI), 0.19 mol of2,4-tolylene diisocyanate (TDI), and 0.01 mol of potassium fluoride wereloaded together with N-methyl-2-pyrrolidone to a solids concentration of20% and stirred at 100° C. for 5 hours, and then the resulting mixturewas diluted with N-methyl-2-pyrrolidone to a solids concentration of 14%to synthesize a polyamide-imide resin solution (PI-d). Thepolyamide-imide resin obtained had a logarithmic viscosity of 0.45 dl/gand a glass transition temperature of 315° C.

A battery separator was obtained in the same manner as in Example A-3except that a varnish comprising the polyamide-imide resin solution(PI-d) in place of the polyamide-imide resin solution (PI-a) was used.

Comparative Example A-4

A battery separator was obtained in the same manner as in Example A-3except that a polypropylene porous membrane having a thickness of 10.0μm, a porosity of 45%, an average pore size of 0.15 μm, and an airresistance of 450 sec/100 cc Air was used as a porous membrane A, andthe absolute humidity was 18.4 g/m³ in both the low humidity zone andthe high humidity zone.

Comparative Example A-5

A battery separator was obtained in the same manner as in Example A-3except that the amount of the varnish was adjusted to a final thicknessof 14.0 μm.

TABLE 1 Thickness of Air permeability Viscosity Absolute porous ofporous Particle of heat humidity of membrane A membrane A size resistantParticle low humidity (μm) (sec/100 ccAir) Particle type (μm) resinratio zone (g/m³) Example A-1 9.0 450 Alumina 0.50 1.35 90.3% 1.8Example A-2 9.0 450 Alumina 0.50 1.35 90.3% 4.0 Example A-3 9.0 450Alumina 0.50 1.35 90.3% 5.5 Example A-4 9.0 450 Alumina 0.50 1.35 90.3%5.5 Example A-5 9.0 450 Alumina 0.50 1.35 90.3% 5.5 Example A-6 9.0 450Alumina 0.50 1.35 80.5% 5.5 Example A-7 9.0 450 Alumina 0.50 1.35 95.1%5.5 Example A-8 9.5 300 Alumina 0.50 1.35 90.3% 5.5 Example A-9 7.0 220Alumina 0.50 1.35 90.3% 5.5 Example A-10 5.0 200 Alumina 0.50 1.35 90.3%5.5 Example A-11 8.0 450 Alumina 0.50 1.35 90.3% 5.5 Example A-12 9.0450 Alumina 0.50 1.35 90.3% 5.5 Example A-13 9.0 450 Alumina 0.50 0.8590.3% 5.5 Example A-14 9.0 450 Titania 0.38 1.35 90.3% 5.5 Example A-159.0 130 Alumina 0.50 1.35 90.3% 5.5 Example A-16 9.0 450 Crosslinked2.50 1.35 90.3% 5.5 polymer particle Example A-17 9.0 450 Alumina 0.501.35 90.3% 5.5 Example A-18 9.0 400 Alumina 0.50 1.35 90.3% 5.5Comparative 9.0 450 Alumina 0.50 1.35 90.3% 7.0 Example A-1 Comparative9.0 450 Alumina 0.50 1.35 90.3% 5.5 Example A-2 Comparative 9.0 450Alumina 0.50 1.35 90.3% 5.5 Example A-3 Comparative 10.0 450 Alumina0.50 0.45 90.3% 18.4 Example A-4 Comparative 9.0 450 Alumina 0.50 1.3590.3% 5.5 Example A-5 Absolute Thickness humidity of Rising range of airVariation of battery Peeling high humidity resistance [Y-X] of airseparator strength Curing zone (g/m³) (sec/100 ccAir) resistance (μm)(N/25 mm) (mm) Example A-1 12.0 60 43 11.5 1.8 6 Example A-2 12.0 55 4511.5 1.7 6 Example A-3 12.0 52 42 11.5 1.5 5 Example A-4 7.0 70 47 11.51.9 7 Example A-5 16.0 38 30 11.5 1.3 4 Example A-6 12.0 69 48 11.5 2.38 Example A-7 12.0 54 43 11.5 1.5 3 Example A-8 12.0 67 42 11.5 2.2 5Example A-9 12.0 38 41 11.5 1.6 7 Example A-10 12.0 33 40 11.5 1.4 8Example A-11 12.0 40 28 10.5 1.3 3 Example A-12 12.0 80 46 11.5 2.3 4Example A-13 12.0 59 45 11.5 2.7 4 Example A-14 12.0 60 47 11.5 1.8 6Example A-15 12.0 25 13 11.5 1.9 7 Example A-16 12.0 52 43 11.5 2.0 8Example A-17 12.0 31 45 11.5 2.2 6 Example A-18 12.0 31 46 11.5 2.1 6Comparative 12.0 32 38 11.5 0.4 3 Example A-1 Comparative 5.0 105 7311.5 2.4 10 Example A-2 Comparative 12.0 60 87 11.5 0.9 7 Example A-3Comparative 18.4 58 48 11.5 0.8 5 Example A-4 Comparative 12.0 63 5014.0 1.8 11 Example A-5 Note) Crosslinked polymer particle:Crosslinkedpolymethylmethacrylate particle

Table 1 shows the properties of Examples A-1 to 18 and ComparativeExamples A-1 to 5.

Example B-1 Preparation of Varnish

KF polymer #1120 (polyvinylidene fluoride available from KUREHACORPORATION (melting point 175° C.), 12% N-methylpyrrolidone solution)was used as a fluororesin solution.

The polyvinylidene fluoride resin solution, alumina particles having anaverage diameter of 0.50 μm, and N-methyl-2-pyrrolidone were mixed at aweight ratio of 26:34:40, and the resulting mixture was placed into apolypropylene container together with zirconium oxide beads(“Torayceram” (registered trademark) beads available from TORAYINDUSTRIES, INC., diameter: 0.5 mm) and dispersed for 6 hours using apaint shaker (manufactured by Toyo Seiki Seisaku-Sho, Ltd.). Thedispersion was then filtered through a filter with a filtration limit of5 lam to prepare a varnish.

The varnish was applied to a porous membrane A (polypropylene,thickness: 9.0 μm, porosity: 40%, average pore size: 0.10 μm, and airresistance: 450 sec/100 cc Air) by blade coating. The coated membranewas passed through a low humidity zone at a temperature of 25° C. and anabsolute humidity of 1.8 g/m³ over 8 seconds, and then through a highhumidity zone at a temperature of 25° C. and an absolute humidity of12.0 g/m³ over 5 seconds, immersed in an aqueous solution containing 5%by weight of N-methyl-2-pyrrolidone for 10 seconds, washed with purewater, and then passed through a hot-air drying furnace at 70° C. fordrying to obtain a battery separator having a final thickness of 11.5μm.

Example B-2

A battery separator was obtained in the same manner as in Example B-1except that the absolute humidity in the low humidity zone was 4.0 g/m³.

Example B-3

A battery separator was obtained in the same manner as in Example B-1except that the absolute humidity in the low humidity zone was 5.5 g/m³.

Example B-4

A battery separator was obtained in the same manner as in Example B-3except that the absolute humidity in the high humidity zone was 7.0g/m³.

Example B-5

A battery separator was obtained in the same manner as in Example B-3except that the absolute humidity in the high humidity zone was 16.0g/m³.

Example B-6

A battery separator was obtained in the same manner as in Example B-3except that the mixing ratio of the polyvinylidene fluoride resinsolution, alumina particles having an average diameter of 0.50 μm, andN-methyl-2-pyrrolidone used in Example B-1 was 26:15:59 (weight ratio).

Example B-7

A battery separator was obtained in the same manner as in Example B-3except that the mixing ratio of the polyvinylidene fluoride resinsolution, alumina particles having an average diameter of 0.50 μm, andN-methyl-2-pyrrolidone used in Example B-1 was 18:41:41 (weight ratio).

Example B-8

A battery separator was obtained in the same manner as in Example B-3except that a polypropylene porous film having a thickness of 9.5 μm, aporosity of 40%, an average pore size of 0.15 μm, and an air resistanceof 300 sec/100 cc Air was used as a porous film A.

Example B-9

A battery separator was obtained in the same manner as in Example B-3except that a polypropylene porous film having a thickness of 7.0 μm, aporosity of 40%, an average pore size of 0.15 μm, and an air resistanceof 220 sec/100 cc Air was used as a porous film A.

Example B-10

A battery separator was obtained in the same manner as in Example B-3except that a polypropylene porous film having a thickness of 5.0 μm, aporosity of 40%, an average pore size of 0.15 μm, and an air resistanceof 200 sec/100 cc Air was used as a porous film A.

Example B-11

A battery separator was obtained in the same manner as in Example B-3except that the amount of the varnish was adjusted to a final thicknessof 10.5 μm.

Example B-12

A battery separator was obtained in the same manner as in Example B-3except that a varnish comprising titanium oxide particles (availablefrom Titan Kogyo, Ltd., trade name “KR-380”, average particle size: 0.38μm) in place of alumina particles was used.

Example B-13

A battery separator was obtained in the same manner as in Example B-3except that a polypropylene porous film having a thickness of 9.0 μm, aporosity of 38%, an average pore size of 0.15 μm, and an air resistanceof 130 sec/100 cc Air was used as a porous film A.

Example B-14

A battery separator was obtained in the same manner as in Example B-3except that a varnish comprising cross-linked polymer particles(polymethyl methacrylate cross-linked particles (product name: “Epostar”(registered trademark) MA, type 1002, available from NIPPON SHOKUBAICO., LTD., average particle size: 2.5 μm)) in place of alumina particleswas used.

Example B-15

The same varnish as in Example B-1 was applied to a corona-treatedsurface of a polyethylene terephthalate resin film having a thickness of50 μm (E5101 available from TOYOBO CO., LTD.) by blade coating. Thecoated film was passed through a low humidity zone at a temperature of25° C. and an absolute humidity of 1.8 g/m³ over 8 seconds, and thenthrough a high humidity zone at a temperature of 25° C. and an absolutehumidity of 12.0 g/m³ over 5 seconds. After 1.7 seconds, a porous film A(polypropylene, thickness: 9.0 μm, porosity: 45%, average pore size:0.15 μm, and air resistance: 450 sec/100 cc Air) was laminated on thegel-like heat resistant resin surface described above, and the laminatewas placed into an aqueous solution containing 5% by weight ofN-methyl-2-pyrrolidone, washed with pure water, and then passed througha hot-air drying furnace at 70° C. for drying to obtain a batteryseparator having a final thickness of 11.5 μm.

Example B-16

A battery separator was obtained in the same manner as in Example B-3except that a porous membrane having a three-layer structure ofpolypropylene/polyethylene/polypropylene (thickness: 9.0 μm (3.0 μm/3.0μm/3.0 μm), porosity: 40%, average pore size: 0.10 μm, and airresistance: 400 sec/100 cc Air was used as a porous film A.

Comparative Example B-1

A battery separator was obtained in the same manner as in Example B-3except that the absolute humidity in the low humidity zone was 7.0 g/m³.

Comparative Example B-2

A battery separator was obtained in the same manner as in Example B-3except that the absolute humidity in the high humidity zone was 5.0g/m³.

Comparative Example B-3

A battery separator was obtained in the same manner as in Example B-3except that a polypropylene porous membrane having a thickness of 10.0μm, a porosity of 45%, an average pore size of 0.15 μm, and an airresistance of 450 sec/100 cc Air was used as a porous membrane A, andthe absolute humidity was 18.4 g/m³ in both the low humidity zone andthe high humidity zone.

Comparative Example B-4

A battery separator was obtained in the same manner as in Example B-3except that the amount of the varnish was adjusted to a final thicknessof 14.0 μm.

TABLE 2 Thickness of Air permeability Absolute porous of porous Particlehumidity of membrane A membrane A size Particle low humidity (μm)(sec/100 ccAir) Particle type (μm) ratio zone (g/m³) Example B-1 9.0 450Alumina 0.50 90.3% 1.8 Example B-2 9.0 450 Alumina 0.50 90.3% 4.0Example B-3 9.0 450 Alumina 0.50 90.3% 5.5 Example B-4 9.0 450 Alumina0.50 90.3% 5.5 Example B-5 9.0 450 Alumina 0.50 90.3% 5.5 Example B-69.0 450 Alumina 0.50 80.5% 5.5 Example B-7 9.0 450 Alumina 0.50 95.1%5.5 Example B-8 9.5 300 Alumina 0.50 90.3% 5.5 Example B-9 7.0 220Alumina 0.50 90.3% 5.5 Example B-10 5.0 200 Alumina 0.50 90.3% 5.5Example B-11 9.0 450 Alumina 0.50 90.3% 5.5 Example B-12 9.0 450 Titania0.38 90.3% 5.5 Example B-13 9.0 130 Alumina 0.50 90.3% 5.5 Example B-149.0 450 Crosslinked 2.50 90.3% 5.5 polymer particle Example B-15 9.0 450Alumina 0.50 90.3% 5.5 Example B-16 9.0 400 Alumina 0.50 90.3% 5.5Comparative 9.0 450 Alumina 0.50 90.3% 7.0 Example B-1 Comparative 9.0450 Alumina 0.50 90.3% 5.5 Example B-2 Comparative 10.0 450 Alumina 0.5090.3% 18.4 Example B-3 Comparative 9.0 450 Alumina 0.50 90.8% 5.5Example B-4 Absolute Thickness humidity of Rising range of air Variationof battery Peeling high humidity resistance [Y-X] of air separatorstrength Curing zone (g/m³) (sec/100 ccAir) resistance (μm) (N/25 mm)(mm) Example B-1 12.0 59 44 11.5 1.8 5 Example B-2 12.0 54 45 11.5 1.7 6Example B-3 12.0 53 43 11.5 1.6 5 Example B-4 7.0 69 47 11.5 1.8 7Example B-5 16.0 33 31 11.5 1.2 3 Example B-6 12.0 68 46 11.5 2.2 8Example B-7 12.0 55 44 11.5 1.3 2 Example B-8 12.0 67 41 11.5 2.1 5Example B-9 12.0 38 42 11.5 1.5 7 Example B-10 12.0 34 40 11.5 1.3 7Example B-11 12.0 40 29 10.5 1.3 3 Example B-12 12.0 59 48 11.5 1.7 6Example B-13 12.0 24 15 11.5 1.8 6 Example B-14 12.0 51 43 11.5 1.8 8Example B-15 12.0 30 46 11.5 2.2 8 Example B-16 12.0 31 47 11.5 2.0 5Comparative 12.0 32 39 11.5 0.3 3 Example B-1 Comparative 5.0 107 7711.5 2.2 10 Example B-2 Comparative 18.4 58 49 11.5 0.8 5 Example B-3Comparative 12.0 64 53 14.0 1.6 12 Example B-4 Note) Crosslinked polymerparticle:Crosslinked polymethylmethacrylate particle

Table 2 shows the properties of Examples B-1 to 17 and ComparativeExamples B-1 to 4.

INDUSTRIAL APPLICABILITY

The battery separator of the present invention is a battery separatorthat has a balance between excellent adhesion of a heat resistant resinlayer and a small amount of air resistance increase even when membranesbecome thinner and thinner in the future, and is suitable for increasein battery capacity, excellent ion permeability, and high-speedprocessability in a battery assembly process.

1. A battery separator comprising: a porous membrane A comprising apolypropylene resin, and a porous membrane B laminated thereoncomprising a heat resistant resin and inorganic particles orcross-linked polymer particles, wherein the porous membrane A satisfiesexpressions (a) to (c), the battery separator satisfies expressions (d)to (f), and the heat resistant resin comprises a fluororesin:t(A)<10 μm  (a) wherein t (A) is a thickness of the porous membrane A;0.01 μm≦R(A)≦1.0 μm  (b) wherein R (A) is an average pore size of theporous membrane A;30%≦V(A)≦70%  (c) wherein V (A) is a porosity of the porous membrane A:t(T)≦13 μm  (d) wherein t (T) is a thickness of the entire batteryseparator;F(A/B)≧1.0 N/25 mm  (e) wherein F (A/B) is a peeling strength at aninterface between the porous membrane A and the porous membrane B; and20≦Y−X≦100  (f) wherein X is an air resistance (sec/100 cc Air) of theporous membrane A, and Y is an air resistance (sec/100 cc Air) of thebattery separator.
 2. The battery separator according to claim 1,wherein the battery separator has an air resistance of 50 to 600 sec/100cc Air.
 3. (canceled)
 4. (canceled)
 5. A process of producing thebattery separator according to claim 1, comprising steps (i) and (ii):(i): applying a varnish comprising a heat resistant resin whichcomprises a fluororesin and inorganic particles or cross-linked polymerparticles to a porous membrane A comprising a polypropylene resin, andthen passing the porous membrane A through a low humidity zone at anabsolute humidity of not less than 0.5 g/m³ and less than 6.0 g/m³ and ahigh humidity zone at an absolute humidity of not less than 7.0 g/m³ andless than 25.0 g/m³ to form a heat resistant resin membrane on theporous membrane A; and (ii): immersing the composite membrane obtainedin step (i), in which the heat resistant resin membrane is laminated, ina coagulation bath to convert the heat resistant resin membrane into aporous membrane B, and washing and drying the porous membrane B toobtain the battery separator.
 6. A process of producing the batteryseparator according to claim 2, comprising steps (i) and (ii): (i):applying a varnish comprising a heat resistant resin which comprises afluororesin and inorganic particles or cross-linked polymer particles toa porous membrane A comprising a polypropylene resin, and then passingthe porous membrane A through a low humidity zone at an absolutehumidity of not less than 0.5 g/m³ and less than 6.0 g/m³ and a highhumidity zone at an absolute humidity of not less than 7.0 g/m³ and lessthan 25.0 g/m³ to form a heat resistant resin membrane on the porousmembrane A; and (ii): immersing the composite membrane obtained in step(i), in which the heat resistant resin membrane is laminated, in acoagulation bath to convert the heat resistant resin membrane into aporous membrane B, and washing and drying the porous membrane B toobtain the battery separator.